Portable electronic gas detectors are well known means of monitoring for hazardous gases in various environments. Such detectors typically include one or more gas sensors, and electronics to convert the output signal from each sensor into one or more signals representative of the concentration of the gas being monitored. Many types of gas sensors are well known in the art, including for example, electrochemical gas sensors for toxic or asphyxiate gases, catalytic bead (pellistor) gas sensors for combustible gases, and galvanic gas sensors for Oxygen.
FIGS. 1, 1A illustrate a known multi-gas detector 10. Detector 10 includes a housing 12 which carries a plurality 14 of electro-chemical gas sensors 14a, b, c, d. Each of the sensors includes a gas inflow port such as 14a-1. Each of the sensors is covered with a gas permeable membrane such as 14a-2 as would be understood by those of skill in the art to protect the respective sensor from inflowing particulate matter.
Housing 12 can also carry a visual display 16, with perhaps an output port 16-1 for an audio and/or visual alarm. Control circuitry, illustrated in phantom at 18, can be coupled to the sensors 14, display 16 and associated audio/visual output device to provide visual and audio indicators as to a concentration of one of more sensed gases. Control element 20 can be used to select a gas, or gasses to detect and evaluate.
FIG. 2 illustrates a known gas sensor 14i. Sensor 14i can be any one of known electrochemical gas sensors as would be understood by those of skill in the art. Sensor 14i includes an input port 14i-1 formed in a housing 14i-3. The inflow port 14i-1 is covered by a gas permeable membrane 14i-2. Multiple electrodes, for example, three electrodes, are electrically connected to signal pins 14i-4 extending from housing 14i-3 and can be coupled to control circuits such as 18.
Although much progress has been made in improving the performance and robustness of gas sensors, such as the sensors of the plurality 14, a number of problems persist. Gas detectors, such as detector 10 are often used in harsh environments where the gas sensors may be exposed to chemicals in the atmosphere that damage, interfere with, or alter the response of a gas sensor to the gas it is intended to sense.
By way of example, typical chemistries for electrochemical gas sensors for detection of hydrogen sulfide gas often exhibit cross-sensitivity to ethanol or methanol vapors, and typical electrochemical carbon monoxide sensors exhibit cross-sensitivity to hydrogen. It is also known that high concentrations of certain solvent vapors can impair the function of an electrochemical gas sensor by promoting electrolyte flooding within the porous electrode structure or by promoting wetting of the supporting diffusion membrane materials of the electrochemical cell.
In general these types of interactions are problematic. They can lead to a number of undesirable outcomes including inaccurate measurements of gas concentration, increased probability of false alarms, reduced sensitivity, delayed sensor responsiveness to a target gas, or even to permanent impairment or failure of the gas sensor.
In practice, manufacturers recommend regular verification of sensor operation by performing a functional test (commonly referred to as a bump test) in which the gas sensors are exposed to a gas mixture of known, fixed composition of one or more analyte gases that the detector is designed to detect. Typically, the concentration of analyte in the test gas is sufficient to trigger an alarm response in a properly functioning gas detector, thereby providing verification of a sensor's response to gas as well as functional verification of the alarm indicating means disposed within the gas detector.
Additionally, a number of electrical sensor interrogation techniques have been developed to provide a means of evaluating the condition of electrochemical gas sensors. These methods are able to identify a number of common failure modes inside the sensor, including for example, loss of electrical continuity within the sensor, or changes in the internal resistance of the sensor.
While these advances offer some improvements in the ability to verify some aspects of sensor functionality, they do not provide a means to alert the user to the presence of gases in the atmosphere that could be causing inaccurate readings or damage to the gas sensors while the detector is operating.
It is thus desirable to develop improved means for detecting and indicating the presence of chemicals or compounds that can impair the performance or function of a gas detector while it is operational and monitoring the atmosphere.
Colorimetric means of gas detection and indication are also known in the art. Examples of available calorimetric gas detection products include Draeger Tubes, available from Draeger Safety of Germany, and Chemcassette Tapes, available from Honeywell Analytics (formerly MDA Scientific) of the United States. Colorimetric techniques and chemistries have been developed over the years to detect several hundred different chemicals and compounds.
Typically the chemical formulation of a colorimetric sensing material is developed to produce a material that will undergo a lasting color change upon exposure to and chemical reaction with the analyte of interest. The persistence of the change in color of the sensing material is desirable in these applications since it can be used to provide a physical record of the chemical measurement being taken.